The present invention relates to a sealing device which prevents the leakage flow between a rotating element and a stationary member, and a fluid machine which provides the sealing device.
As has the vertical sectional view of its upper half part illustrated in FIG. 7, a multistage centrifugal compressor being one example of a multistage type fluid machine operates so that a working gas 10 drawn through an intake pipe 7 is compressed and then discharged out of the machine through a delivery pipe 8 as a plurality of stages of impellers 6 (6a.about.6f) mounted on a rotary shaft 1 rotate. More specifically, after the working gas 10 has flowed in through the intake pipe 7, its pressure is raised by the impellers 6 at the respective stages. Subsequently, pressure recovery is achieved in passing through diffusers 13 (13a.about.13f) and return channels 14 (14a.about.14e) disposed at the respective stages, whereupon it passes to the delivery pipe 8. Labyrinth seals 11 and 12 are installed between the rotary shaft 1 as well as the impellers 6 and a stationary side casing 9. These labyrinth seals include the labyrinth seals 11 fitted at parts at which the working gas 10 returns from the outlet sides of the respective impellers 6 to the inlet sides thereof, in other words, which are near the inlets of the respective impellers 6 (hereinbelow, the labyrinth seals 11 shall be termed the "inlet labyrinth seals"), and the labyrinth seals 12 fitted between the respectively adjacent two compressor stages (hereinbelow, the labyrinth seals 12 shall be termed the "interstage labyrinth seals"). Further, a labyrinth seal 15 is used at the part of a balance drum.
FIG. 8 illustrates the details of the labyrinth seals which have heretofore been employed in the multistage centrifugal compressor shown in FIG. 7. Referring to FIG. 8, each of the labyrinth seals forms cylindrical sealing surfaces which are parallel to the rotary shaft 1. Also, either the rotating side member or the stationary side member (here in the illustration of FIG. 8, the stationary side member) is provided with a plurality of fins 2 corresponding to each labyrinth seal, and the gaps between the distal ends of the fins 2 and the opposing surface are narrowed, thereby suppressing the corresponding one of leakage streams 5a and 5b of the working gas 10 from the high-pressure side of the entire labyrinth sealing device.
Examples wherein such labyrinth seals are disposed, are stated in the official gazettes of Japanese Patent Applications Laid-open No. 217595/1995 and No. 203565/1992. The example in No. 217595/1995 is intended to reduce the rate of leakage flow in such a way that a stationary side member is provided with fins, the distal ends of which are subjected to gap-forming coating (hereinbelow, the gap-forming coating shall be termed the "abradable coating") to-be-bitten, thereby reducing the clearance between the stationary side member and a rotating side member. On the other hand, the example in No. 203565/1992 is intended to reduce the rate of leakage flow in such a way that a rotating side member is provided with fins, while a stationary side member is subjected to abradable coating to-be-bitten, thereby to reduce the clearance between both the members.
Examples in each of which the parts of a stationary side member corresponding to the inlet parts of impellers are similarly subjected to abradable coating to-be-bitten with the intention of reducing the rate of leakage flow, are also reported in Mitsubishi Heavy Industries Technical Review, Vol. 23, No. 5 (1986-9), and Proceedings of Ebara, No. 154 (1992-1).
The labyrinth sealing device in the prior art is so designed that the fins provided on the stationary side member do not touch the rotating side member in principle, but define the gaps with respect to the rotating side member without fail. With this device, therefore, the reduction of the clearance between both the members is limited. By way of example, in a case where the flow rate of the leakage streams 5a and 5b is small relative to the flow rate of the mainstream 10 in the illustration of FIG. 8, the performance of the fluid machine is little affected by these leakage streams. However, at the low specific speed stage, the operating efficiency of the fluid machine is drastically reduced due to the large leakage flow rate. Moreover, when the clearance is made excessively small in the known labyrinth sealing device, it is apprehended in the case of, for example, the compressor that unstable vibrations ascribable to the rotating stall or to surge will arise to damage the rotating shaft 1 on account of being touched by the labyrinth fins.
On the other hand, regarding the technique stated in the official gazette of Japanese Patent Application Laid-open No. 217595/1995 or No. 203565/1992 wherein, in order to enhance a sealing effect, the smallest possible clearance is defined between sealing surfaces, one of which is subjected to the abradable coating to-be-bitten, it is apprehended that the layer of the abradable coating to-be-bitten will degrade and will fail to demonstrate an expected performance over a long term. More specifically, when the technique is applied to a processing compressor or the like which is treated with various kinds of gases, the material of the coating degrades due to any corrosive gas. Thereafter, when the fluid instability phenomenon such as the surging or the rotating stall has taken place, the vibrations of a shaft increase to bring the fins and the coating surface into contact. As a result, the coating material might peel off. In this case, the performance of the fluid machine is drastically lowered. Another drawback is that the reliability of the fluid machine decrease.